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The oxidation of carbon monoxide by oxygen over platinum, palladium and rhodium catalysts from 10−10 to 1 bar
Chemical Engineering and Processing, 33 (1994) 363-369
ELSEVIER
The oxidation of carbon monoxide by oxygen over platinum, palladium and rhodium catalysts from 10-‘” to 1 bar S. Fuchs, T. Hahn *, H.-G. Lintz Institut
ftir Chemische Verfahrenstechnik der Uninersitiit Karlsruhe, D- 76128 Karlsruhe, Germany
Abstract The noble metal catalysed oxidation of CO by O2 has been studied on polycrystalline foils, ribbons and wires of Pt, Pd and Rh as well as on a variety of supported Pt catalysts. At temperatures from 180 to 600 “C, the partial pressure ratio was changed over the range 0.01
Platinum catalyst; Palladium Rate constants; Mass-transfer
1. Introduction Since the classical work of Langmuir [ 11, the oxidation of carbon monoxide by oxygen catalysed by noble metals has been studied in great detail. The considerable interest in this simple, bimolecular reaction may be associated with the assumption that it should provide a suitable model for research on fundamental problems in
heterogeneous catalysis. The experimental conditions employed in the studies described in the literature cover a broad range. Thus, with regard to the total pressure it is possible to divide the experimental work into two groups: (a) studies at high or ultra-high vacuum conditions (P < 10d4 mbar) over model catalysts; and (b) studies at (or near) atmospheric pressure mainly over supported catalysts. The difference between the two approaches represents the so-called ‘technological gap’
* Corresponding
author.
0255-2701/94/%7.00 Q 1994 SSDI 0255-2701(94)02007-H
Elsevier Science S.A. All rights reserved
catalyst; Rhodium coefficients
catalyst; Zirconia
support;
Alumina
[2], both with respect to total pressure and to surface structure and composition of the catalyst. In 1985, Stoltze and Nnrskov published a pioneering paper on the extrapolation of kinetic data from ultrahigh vacuum to practical conditions in the case of ammonia synthesis [3]. A few months later another group presented similar work concerning the CO/O, reaction on rhodium catalysts [4], but the study was restricted to conditions where the reaction is inhibited by carbon monoxide. In general, there is experimental evidence that the extrapolation of kinetic data from model experiments to practical conditions is not feasible. This holds even in the case of the rather simple CO oxidation on noble metals. However, in the latter case the catalysts used on both sides of the ‘technological gap’ are quite different, and the variations in temperature and gas phase composition investigated are relatively small. The scope of the present study was therefore to re-examine the oxidtion of carbon monoxide by oxygen on platinum, palla-
S. Fuchs et al. / Chemical Engineering and Processing 33 (1994) 363-369
364
dium and rhodium over a wide range of temperatures and compositions in an attempt to provide a definitive answer to the question as to what extent the extrapolation of kinetic data from model experiments to practical conditions is feasible in the case of this simple reaction.
the geometric area Fgeo of the catalyst metal area F,,,, via the equations:
=-.- 1 gee
dt
(2)
df
where 5 represents Two experimental set-ups were employed: apparatus’ using a differentially (1) A ‘vacuum pumped mass spectrometer over the pressure range lo-* < Preactor (mbar) Q 1. (2) A jet-stirred loop reactor at atmospheric pressure and at 0.3 <:pi (mbar) < 40 [5]. The reactants and balance nitrogen (CO, 99.997 mol%; O,, 99.995 mol%; N,, 99.999 mol%; Messer Griesheim) were fed to the system from cylinders via leak valves or mass flow controllers. Partial pressures were measured in the vacuum apparatus by means of a quadrupole mass spectrometer (QMG 3 11, Balzers). Concentrations were monitored in the recirculation system by means of non-dispersive IR spectrometers (CO: Ultramat, Siemens; COZ: Uras 2T, Hartmann and Braun) and a magnetic device (0,: Magnos, Hartmann and Braun). Two types of catalysts were used: (a) polycrystalline foils, ribbons and wires of platinum, palladium and rhodium (Heraeus); and (b) different platinum catalysts supported on non-porous platelets of Pyrex or quartz glass (Schott), zirconia (Friedrichsfeld) and porous tlalumina. The foils, ribbons and wires were recrystallized in vacuum or in air at temperatures above 800 “C. The supported platinum catalysts were prepared from aqueous tetrammine nitrate, dried (1 h at 70 “C) and calcined (2 h at 500 “C) in air. The dispersion D (in % metal exposed) was determined by standard Hz chemisorption at room temperature. Table 1 lists the data for the supported catalysts, where L corresponds to a characteristic length used later in the calculation of the mass-transfer coefficients. Reaction rates were obtained by mass balance for the open systems in the steady state. The rate was related to
the molar extent of the reaction
co + +o, + co,
(3)
3. Results and discussion No measurable difference between the steady-state catalytic activity for the catalysts heated in vacua or in air and no deactivation ‘of the catalysts could be observed. Thus the reproducible results are representative of noble metal catalysts under proper conditions in both cases. Fig. 1 illustrates the reaction rate r’ obtained on platinum foil at three temperatures, as a function of the partial pressure of carbon monoxide. The different (constant) values of the oxygen partial pressure are indicated in the upper part of the figure. The solid line represents the rate of impingement of the carbon
Fig. 1. Plot of T’ as a function of pco
over platinum foil.
Table 1 Data for supported catalysts Catalyst Pt-0.0 1/ZrO, Pt-l.S/Zro, Pt-lO/Zro, Pt-O.Ol/Al,O, Pt-0.2/Pyrex Pt-lS/Pyrex Pt-0.2/Quartz a Not measurable.
m4 (mg)
0.0089 1.5 10.4 1.0 0.19 1.95 0.135
Fsw (mm’)
or the
(1)
rBcoF
2. Experimental
platelet
Fmctal (cm’)
D (%)
L (mm)
210
_a
_a
392 392 330 381 457 310
30 50 50 -8 30 _a
0.72 0.17 13 _a 0.54 _a
10.0 14.0 14.0 12.5 14.4 14.4 13.0
365
S. Fuchs et al. 1 Chemical Engineering and Processing 33 (1994) 363-369
lo-'
r’ 2
mol~cm-
-se I
10-5
.
x+
. .
/
-!a?zof poz over platinum
foil.
monoxide molecules onto the surface, and quantifies the upper limit of Y’ if every impinging molecule reacted. Fig. 2 shows a similar representation of r’ as a function of the oxygen partial pressure. Due to the stoichiometry defined by Eq. (3), the rate of impingement has been multiplied by a factor of two in order to obtain the upper rate limit represented by the solid line. It is clear from these figures that the well-known formal kinetics for CO oxidation on noble metals remained valid over the whole range of pressures indicated. In oxidizing gas mixtures the rate is first order with respect to pcO and independent of pO,. The temperature dependence was very small over the range 300 < T(C) G 600. In reducing gas mixtures the order of reaction with respect to oxygen was + 1; at sufficiently high temperatures the rate was independent of pcO but was inhibited by the CO excess at ‘low’ temperatures. The reaction order changed at a well-defined value of the partial pressure ratio pco/poz which increased from ‘low’ to ‘high’ values of the total pressure in the system. However, the results indicate a major difference in catalytic activity between the ‘low’ and ‘high’ total pressure region. This is best shown by comparison of the reaction probabilities bi, i.e. ratio of the number of reacting and impinging molecules of species i. At partial pressures pco < 0.1 mbar, bco N 0.13, but over the range pcO > 0.1 mbar a much lower value of bco u 1.3 x 10d3 was obtained. The results were quite similar in the case of palladium and rhodium. This apparent ‘gap’ between ‘high’ and ‘low’ pressure results is well known. Figs. 3 and 4 show a compilation of reliable literature data from platinum and palladium catalysts together with our results at values of CO partial pressure covering a range of 10 orders of magnitude. Our own values obtained at pcO i 5 x 10m2mbar agree with the ultra-high vacuum results of White et al. [6, 151 and Tamaru et al. [ 161 (pcO < 10m5mbar). The same applies to the results reported by Coulston and Haller [7] for pcO = 5 mbar. In oxidizing gas mixtures, 13% (Pt) relative to 15% (Pd) of the impinging CO
0
,
Pm mbar
mbar
Fig. 2. Plot of r’ as a function
PO,
Fig. 3. Plot of r’ as a function ofp Co over different platinum catalysts.
10-4
rl molem-
1
*sI
:*
.
10-s
P'o,.O~'
. . ;/’
..
PC0
mbar
Fig. 4. Plot of r’ as a function catalysts.
of pco
over different
palladium
molecules react to form COZ whereas in reducing gas mixtures 10% (Pt) relative to 25O/u(Pd) of the impinging 0, molecules react to form CO,. However, over the range pcO > 5 x lo-* mbar the catalytic activities reported vary over approximately five orders of magnitude. The highest activities were obtained on foils (Pt) or single crystals (Pd), but the corresponding reaction probabilities remained approximately two orders of magnitude below those obtained at low pressures. Similar findings were obtained in the case of rhodium. What are the reasons for the dramatic differences observed between the ‘low’ and ‘high’ pressure results, a fact known equally for the HZ/O, reaction on noble metal catalysts [22-24]? Several suggestions have been made in the literature: (1) Different mechanisms for ‘low’ and ‘high’ pressures [25-321. (2) Different phase compositions (especially of oxides or surface oxides) [33-391. (3) Influence of the support [40-421. (4) Influence of surface structure (particle size) [lo, 14, 30-32,43-511. (5) Influence of contaminants [ 131. To reduce the number of influencing variables, our experiments were initially restricted to polycrystalline
S. Fuchs et al. 1 Chemical Engineering and Processing 33 (1994) 363-369
366
lO-“oW Fig. 5. Plot of r’ as a function
JQ_ mbar
100 mbor
of pco over Pt, Pd and Rh foils
foils. Contamination at higher pressures was ruled out by the use of a jet-stirred loop reactor without moving parts [5]. As a result, the observed activities remained constant for more than 1 week. Additionally, the simple geometry of the catalysts and the well-defined gas flow made it easy to account for mass-transport limitations, a fact often neglected in studies reported in the literature. Fig. 5 shows the measured reaction rates obtained over different noble metal foils as well as the calculated lines for maximum molar flow to the catalyst surface, i.e. 1 ~
F SO
(4)
4. max = e:, max = bi, bulk
of component i (CO; where ci, bulk is the concentration 0,) in the bulk of the gas phase and /I is an appropriate mass-transfer coefficient obtained via the Sherwood number [5]. Thus the horizontal line corresponds to constant maximum flow of oxygen and the inclined line corresponds to the maximum flow of carbon monoxide which is dependent upon the CO partial pressure. It is obvious that the reaction rates measured are limited by mass transfer. This assumption can be verified by varying the characteristic length L of the catalysts as shown in Fig. 6 for the different platinum catalysts. Even if the value of L is reduced from 10 mm to 0.16 mm, i.e. /I is enhanced from 29 to 205 cm s-l the measured reaction rates remain mass transfer limited.
Fig. 7. Plot of r’ as a function structure.
of pco:
”
Fig. 6. Plot of r’ as a function lengths.
'.,,.,I 1
of pco:
",,',.I PO0 mbor
variation
10
of characteristic
and
This striking but somewhat trivial result is not altered by the use of various supported catalysts (Fig. 7) even if the surface of the supported platelet is not totally covered by the noble metal (catalyst Pt-O.Ol/ZrO,). The conclusion also remains valid at lower temperatures; even if the temperature was decreased to 180 “C (at lower temperatures the conversions in the recirculation system were too low to be accurately determined), the measured rates were clearly transport limited as long as they remained first order with respect to the limiting component. The results thus indicate that the sink at the surface created by the chemical reaction is sufficiently strong to make the reaction transport limited in every case. It is interesting to note that the case of the supported catalyst which is uncompletely covered by the active component has been investigated theoretically and experimentally by Schhinder et al. [ 52, 531 for H, oxidation on platinum. It follows, therefore, that any evaluation of the results must take into account the coupling of mass transfer and chemical reaction. If both steps are first order dependent in concentration, i.e. over the range where the reaction rate depends linearly on the limiting component, the intrinsic rate constant k’ can be calculated from the experimentally determined value k:,, and the estimated transport coefficient p via x cc0 -k’ - exp cc0
lo-'! 0.1
influence of support
(5)
The calculated values of either k’ or b,, represent only lower limits but the experimental results obtained on the catalysts with the smallest characteristic length, i.e. the highest values of /?, lead to values of b,, which compare favourably to the sticking probabilities for CO absorption determined under high vacuum conditions (Pt wire, L =O.l6mm, b,= 0.37; Pd wire, L = 0.16 mm, bco = 0.24; Rh foil, L = 10 mm, bco = lo-‘). This result clearly indicates that under oxidizing conditions and at temperatures above 180 “C, at least in the case of Pt and Pd, the catalytic activity is controlled by
S. Fuchs et al. 1 Chemical Engineering and Processing 33 (1994) 363-369
the same elementary steps as in the ultra-high vacuum model experiments, viz. the rate of adsorption of the limiting compound CO. In the case of rhodium, the interpretation of the results is less clear but the rates obtained are undoubtedly higher than those reported by Goodman et al. [4,9, 54, 551 for single crystal surfaces. Under a reducing atmosphere and at low temperatures, i.e. the region of inhibition by CO, mass-transfer resistances do not interfere with the measured rates. Over that range, it is therefore possible to compare the results obtained with catalysts of various structure and support. If the rates can be related to the accessible surface of the noble metal, the activities measured on all catalysts of the same metal coincide within a factor of two, Thus no significant influence of catalyst structure and support has been detected. In this context it is interesting to look at the elementary reaction sequence of the CO oxidation on noble metals. It is generally accepted that CO oxidation proceeds via a Langmuir-Hinshelwood step, i.e. the reaction rate is proportional to the surface coverage by both carbon monoxide (O,,) and oxygen (Go), i.e. r LH
=
%etai ~ . kLH a,-, N
e.
(6)
= kzH exp g (7) ( > From a knowledge of the intrinsic rate, it is possible to specify a lower limit for the Langmuir-Hinshelwood most workers have rate coefficient kLH. Unfortunately only been interested in activation energies and the pre-exponential factor k% merely as an adjustable parameter. It is therefore relatively difficult to compare the limiting value of ktH estimated in our study to the literature values. However, it can be stated that for the usual range of activation energies reported, i.e 90 < ELH (kJ mall’) < 130, the corresponding values of the pre-exponential factor must lie in the range 4 x 10” < ktH (SC’) < 2 x 1016. k,,
4. Conclusions
Detailed measurements of CO oxidation by oxygen on various noble metal catalysts over a broad range of pressure and temperature lead to two important conclusions: (1) Over the range of gas composition and temperature where the rate of reaction is zero order with respect to the component in excess and first order with respect to the limiting reactant, the intrinsic rate coefficients determined under ultra-high vacuum conditions remain valid. (2) Over the range of CO inhibition, neither the catalyst structure nor support have any influence on the reaction, at least in the case of platinum.
367
Thus the apparent gap between the rates of CO oxidation obtained in model experiments and under ambient pressure must be attributed to the influence of mass-transfer limitations.
Nomenclature b
r
reaction probability, concentration, mol mm3 dispersion, activation energy, kJ mol-’ area, m2 rate coefficient length, m mass, kg density of surface metal atoms (1.25 x 10” m-‘) Loschmidt number (6.023 x 10z3 mol-‘) partial pressure, Pa total pressure, Pa reaction rate, mol cmm2 s-i
P 0
mass-transfer coefficient, m s-i surface coverage, -
L E F k L
m n
N P
P
Subscripts geo i
LH
geometric arbitrary species i Langmuir -Hinshelwood
References [I] I. Langmuir, The mechanism of the catalytic action of platinum in the reactions 2C0 + 0s = 2C0, and 2H, + 0, = 2H,O, Trans. Faraday Sot., 17 (1922) 621. [2] H.-P. Bonzel, Ein Thema fur die Oberthichenphysik: Heterogene Katalyse, Phys. BI., 32 (1976) 392. [3] P. Stoltze and J.K. Norskov, Bridging the ‘pressure gap’ between ultrahigh-vacuum surface physics and high-pressure catalysis, Phys. Reo. Len., 55 (1985) 2502. [4] S.H. Oh, G.B. Fisher, J.E. Carpenter and D.W. Goodman, Comparative kinetic studies of CO-O, and CO-NO reactions over single crystal and supported rhodium catalysts, J. Catal., 100 (1986) 360. [S] S. Fuchs and T. Hahn, Construction and application of a jet-stirred loop reactor, Chem. Eng. Process., 32 (1993) 225. [6] A. Golchet and J.M. White, Rates and coverages in the low pressure Pt-catalyzed oxidation of carbon monoxide, J. Carol., 53 (1978) 266. [7] G.W. Coulston and G.L. Haller, Kinetics and mechanism of carbon monoxide oxidation on platinum, palladium and rhodium foils, in D.W. Dweyer and F.M. Hoffmann (eds.), Surfbce Science and Catalysis, ACS Symp. Ser. No. 482, Am. Chem. Sot., Washington, DC, 1992, Chapter 4. [S] E. Hlfele, Thesis, University of Karlsruhe, 1986. [9] P.J. Berlowitz, C.H.F. Peden and D.W. Goodman, Kinetics of CO oxidation on single-crystal Pd, Pt and Ir, J. Phys. Chem., 92 (1988) 5213.
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[lo] E. McCarthy, J. Zahradnik, G.C. Kuczinsky and J.J. Carberry, Some unique aspects of CO oxidation on supported Pt, J. Catal., 39 (1975) 29. [ 1 l] G.S. Za8ris and R.J. Gorte, CO oxidation on Pt/a-Al,O,(OOOl): Evidence for structure sensitivity, J. Cata!., 140 (1993) 418. [ 121 R.K. Herz and S.P. Marin, Surface chemistry models of carbon monoxide oxidation on supported platinum catalysts, J. Catal., 65 (1980) 281. [ 131 N.W. Cant, P.C. Hicks and B.S. Lennon, Steady-state oxidation of carbon monoxide over supported noble metals with particular reference to platinum, J. Catal., 54 (1978) 372. [ 141 N.W. Cant, Metal crystallite size effects and low-temperature deactivation in carbon monoxide oxidation over platinum, J. Cam/., 62 (1980) 173. [ 151 J.R. Creighton, F.-H. Tseng, J.M. White and J.S. Turner, Numerical modeling of steady-state carbon monoxide oxidation on Pt and Pd, J. Phys. Chem., 85 (1981) 703. [ 161 M. Kawai, T. Onishi and K. Tamaru, Oxidation of CO on a Pd surface: application of IR-RAS to the study of the steady-state surface reaction, Appl. Surf. Sci., 8 (1981) 361. [ 171 A.D. Logan and M.T. Paffett, Steady state CO oxidation kinetics over the Pd(100) single crystal surface and the ~(2 x 2)-Sn/ Pd( 100) bimetallic surface alloy, J. C&al., 133 (1992) 179. [18] S.M. Landry, R.A. Dalla Betta, J.P. Lii and M. Bondart, Catalytic oxidation of carbon monoxide on palladium. I. Effect of pressure, J. Phys. Gem., 94 (1990) 1203. [19] K.I. Choi and A. Vannice, CO oxidation over Pd and Cu catalysts. III. Reduced Al,O,-supported Pd, J. Catal., 131(1991) [20] k.L. Goldsmith, Thesis, MIT, 1996. [21] S.N. Pavlova, V.A. Sazonov and V.V. Popovskii, Influence of the nature of support on the properties of Pd-containing catalysts for CO oxidation by dioxygen, React. Kinet. Catal. L&t., 37 (1988) 325. [22] M. Boudart, D.M. Collins, F.V. Hanson and W.E. Spicer, Reactions between H, and 0s on pt at low and high pressures: a comparison, J. Vat. Sci. Technol., 14 (1977) 441. [23] V.P. Zhdanov, VI. Sobolev and V.A. Sobyanin, The steady-state kinetics of the hydrogen-oxygen reaction on a Pt(ll1) surface at low and moderate pressures, Surf. Sci., I75 (1986) L747. [24] L.-G. Petersson and U. Ackelid, Kinetic studies of diffusion limited gas-surface reactions by spatially resolved gas sampling: reaction rates and sticking coefficients in the H, + 0, reaction on Pt, Surf. Sci., 269/270 (1992) 500. [25] N. Pacia, A. Cassuto, A. Pentenero and B. Weber, Molecular beam study of the mechanism of carbon monoxide oxidation on platinum and isolation of elementary steps, J. Catal., 41 (1976) 455. [26] V.L. Kuchayev, Investigation of intermediates of the reaction of CO with 0, on Pt by the ion-ion emission technique, Proc. 6th Int. Congr. Catal., R. Sot. Chem., London, 1976, p. 346. [27] R.C. Yeates, J.E. Turner, A.J. Gellman and G.A. Somorjai, The oscillatory behavior of the CO oxidation reaction at atmospheric pressure over platinum single crystals: surface analysis and pressure dependent mechanisms, Surf. Sci., J49 (1985) 175. [28] B.K. Cho and C.J. Stock, Dissociation and oxidation of carbon monoxide over Rh/Al,O, catalysts, J. Catal., II 7 (1989) 202. [29] C.H.P. Peden and J.E. Houston, Evidence for a carbonate species during CO oxidation on deactivated Rh( 11 I), J. Caral., 1.28 (1991) 405. [30] S. Ladas, H. Poppa and M. Boudart, The adsorption and catalytic oxidation of carbon monoxide on evaporated palladium particles, Surf. Sci., 10.2 (1981) 151. [31] M. Boudart and F. Rumpf, The catalytic oxidation of CO and structure insensitivity, React. Kinet. Catal. Lett., 35 (1987) 95. [32] L. Kieken and M. Boudart, Rate of oxidation of CO on Pd at pressures between 10-l and lo* mbar, Catal. Lett., 17 (1993) 1.
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[33] G. Ertl, Reactive transformation of surface structure, Ser. Bunsetzges. Phys. Chem., 90 (1986) 284. [34] J.E. Turner, B.C. Sales and M.B. Maple, Oscillatory oxidation of CO over a Pt catalyst, Surf. Sci., 103 (1981) 54, Oscillatory oxidation of CO over Pd and Ir catalysts, Surf. Sci., 109 (1981) 591; The oxidation and CO reduction kinetics of a platinum surface, Surf. Sci., 112 (1981) 272; Oscillatory oxidation of CO over Pt, Pd and Ir catalysts: theory, Surf. Sci., 114 (1982) 381. [35] J.T. Kiss and R.D. Gonzalez, Catalytic oxidation of carbon monoxide over Rh/SiO,: an in situ infrared and kinetic study, Ind. Eng. Chem., Prod. Res. Dev., 24 (1985) 216. [36] T.H. Lindstrom and T. T. Tsotsis, Reaction rate oscillations during CO oxidation over Pt/y-Al,03: experimental observations and mechanistic causes, Surf. Sci., 150 (1985) 487; Reaction rate oscillations during CO oxidation over Pt/y-Also,: the issue of propagating waves, Surf. Sci., 167 (1986) L194. [37] K. Otto, L.P. Haack and J.E. De Vries, Identification of two types of oxidized palladium on y-Also1 by XPS, Appl. Cutal. B, I(l992) 1. [38] K. Otto, C.P. Hubbard, W.H. Weber and G.W. Graham, Raman spectroscopy of palladium oxide on y-A&O, applicable to automotive catalysts, Appl. Catal. B, 1 (1992) 317. [39] A. Palazov, G. Kadinov, C. Bonev, R. Dimitrova and D. Shopov, Infrared spectroscopic study of carbon monoxide oxidation over alumina-supported palladium, Surf: Sci., 225 (1990) 21. [40] J. Sarkany and R.D. Gonzalez, Support and dispersion effects on silica- and alumina-supported platinum catalysts: II. Effect on the CO-O, reaction, Appl. Catal., 5 (1983) 85. (411 C.S. Ko and R.J. Gorte, Characterization of oxide impurities on Pt and their etfect on the adsorption of CO and H,, Surf. Sci., 155 (1985) 296. [42] S.H. Oh and C.C. Eickel, Influence of metal particle size and support on the catalytic properties of supported rhodium: COO, and CO-NO reactions, J. Caral., 128 (1991) 526. [43] H. Hopster, H Ibach and G. Comsa, Catalytic oxidation of carbon monoxide on stepped Pt( 111) surfaces, J. Catal., 46 (1977) 37. [44] J.L. Gland, M.R. McClellan and F.R. McFeely, Carbon monoxide oxidation on the kinked Pt(321) surface, J. Chem. Phys., 79 (1983) 6349. [45] A. Szabb, M.A. Henderson and J.T. Yates, Jr., Oxidation of CO by oxygen on a stepped platinum surface: identification of the reaction site, J. Chem. Phys., 6 (1992) 6191. [46] E.C. Akubuiro, X.E. Verykios and L. Lesnick, Dispersion and support effects in carbon monoxide oxidation over platinum, Appl. Catal., 14 (1985) 215. [47] M. Herskowitz, R. Holliday, M.B. Cutlip and C.N. Kenney, Effect of metal dispersion in CO oxidation on supported Pt catalysts, J. Catal., 71 (1982) 408. [48] J.A. Anderson, CO oxidation over alumina supported platinum catalysts, Cutal. Lett., 13 (1992) 363. [49] V. Matolin and E. Gillet, The surface diffusion in CO-oxidation on small supported Pd particles: experimental evidence, Surf. Sci., 266 (1986) Ll15. [50] F. Rumpf, H. Poppa and M. Boudart, Oxidation of carbon monoxide on palladium: role of the alumina support, LMgmuir, 4 (1988) 722. [51] CR. Henry, On the effect of the diffusion of carbon monoxide on the substrate during CO oxidation on supported palladium clusters, Surf. Sci., 223 (1989) 519. [52] E.-U. Schltinder, On the mechanism of the constant drying rate period and its relevance to diffusion controlled gas phase reactions, Chem. Eng. Sci., 43 (1988) 2685. [53] A. Stammer, W. Btlssing and E.-U. Schhinder, Transport phenomena over partially active surfaces, Chem. Eng. Process., 30 (1991) 125.
S. Fuchs et al. I Chemical Engineering and Processing 33 (1994) 363-369 [54] C.H.F. Peden, D.W. Goodman, D.S. Blair, P.J. Berlowitz, G.B. Fisher and S.H. Oh, Kinetics of CO oxidation by 0, or NO on Rh( 111)and Rh( 100) single crystals, J. Phys. Chem., 92 (1988) 1563.
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[55] C.H.F. Peden, P.J. Berlowitz and D.W. Goodman, Kinetics of CO oxidation over a deactivated Rh( 1I I) single crystal catalyst, in M.J. Phillips and M. Ternan (cds.), Proc. 9th Int. Congr. Caral., Chem. Inst. Canada, Ottawa, 1988, p, 1214.
ELSEVIER
The oxidation of carbon monoxide by oxygen over platinum, palladium and rhodium catalysts from 10-‘” to 1 bar S. Fuchs, T. Hahn *, H.-G. Lintz Institut
ftir Chemische Verfahrenstechnik der Uninersitiit Karlsruhe, D- 76128 Karlsruhe, Germany
Abstract The noble metal catalysed oxidation of CO by O2 has been studied on polycrystalline foils, ribbons and wires of Pt, Pd and Rh as well as on a variety of supported Pt catalysts. At temperatures from 180 to 600 “C, the partial pressure ratio was changed over the range 0.01
Platinum catalyst; Palladium Rate constants; Mass-transfer
1. Introduction Since the classical work of Langmuir [ 11, the oxidation of carbon monoxide by oxygen catalysed by noble metals has been studied in great detail. The considerable interest in this simple, bimolecular reaction may be associated with the assumption that it should provide a suitable model for research on fundamental problems in
heterogeneous catalysis. The experimental conditions employed in the studies described in the literature cover a broad range. Thus, with regard to the total pressure it is possible to divide the experimental work into two groups: (a) studies at high or ultra-high vacuum conditions (P < 10d4 mbar) over model catalysts; and (b) studies at (or near) atmospheric pressure mainly over supported catalysts. The difference between the two approaches represents the so-called ‘technological gap’
* Corresponding
author.
0255-2701/94/%7.00 Q 1994 SSDI 0255-2701(94)02007-H
Elsevier Science S.A. All rights reserved
catalyst; Rhodium coefficients
catalyst; Zirconia
support;
Alumina
[2], both with respect to total pressure and to surface structure and composition of the catalyst. In 1985, Stoltze and Nnrskov published a pioneering paper on the extrapolation of kinetic data from ultrahigh vacuum to practical conditions in the case of ammonia synthesis [3]. A few months later another group presented similar work concerning the CO/O, reaction on rhodium catalysts [4], but the study was restricted to conditions where the reaction is inhibited by carbon monoxide. In general, there is experimental evidence that the extrapolation of kinetic data from model experiments to practical conditions is not feasible. This holds even in the case of the rather simple CO oxidation on noble metals. However, in the latter case the catalysts used on both sides of the ‘technological gap’ are quite different, and the variations in temperature and gas phase composition investigated are relatively small. The scope of the present study was therefore to re-examine the oxidtion of carbon monoxide by oxygen on platinum, palla-
S. Fuchs et al. / Chemical Engineering and Processing 33 (1994) 363-369
364
dium and rhodium over a wide range of temperatures and compositions in an attempt to provide a definitive answer to the question as to what extent the extrapolation of kinetic data from model experiments to practical conditions is feasible in the case of this simple reaction.
the geometric area Fgeo of the catalyst metal area F,,,, via the equations:
=-.- 1 gee
dt
(2)
df
where 5 represents Two experimental set-ups were employed: apparatus’ using a differentially (1) A ‘vacuum pumped mass spectrometer over the pressure range lo-* < Preactor (mbar) Q 1. (2) A jet-stirred loop reactor at atmospheric pressure and at 0.3 <:pi (mbar) < 40 [5]. The reactants and balance nitrogen (CO, 99.997 mol%; O,, 99.995 mol%; N,, 99.999 mol%; Messer Griesheim) were fed to the system from cylinders via leak valves or mass flow controllers. Partial pressures were measured in the vacuum apparatus by means of a quadrupole mass spectrometer (QMG 3 11, Balzers). Concentrations were monitored in the recirculation system by means of non-dispersive IR spectrometers (CO: Ultramat, Siemens; COZ: Uras 2T, Hartmann and Braun) and a magnetic device (0,: Magnos, Hartmann and Braun). Two types of catalysts were used: (a) polycrystalline foils, ribbons and wires of platinum, palladium and rhodium (Heraeus); and (b) different platinum catalysts supported on non-porous platelets of Pyrex or quartz glass (Schott), zirconia (Friedrichsfeld) and porous tlalumina. The foils, ribbons and wires were recrystallized in vacuum or in air at temperatures above 800 “C. The supported platinum catalysts were prepared from aqueous tetrammine nitrate, dried (1 h at 70 “C) and calcined (2 h at 500 “C) in air. The dispersion D (in % metal exposed) was determined by standard Hz chemisorption at room temperature. Table 1 lists the data for the supported catalysts, where L corresponds to a characteristic length used later in the calculation of the mass-transfer coefficients. Reaction rates were obtained by mass balance for the open systems in the steady state. The rate was related to
the molar extent of the reaction
co + +o, + co,
(3)
3. Results and discussion No measurable difference between the steady-state catalytic activity for the catalysts heated in vacua or in air and no deactivation ‘of the catalysts could be observed. Thus the reproducible results are representative of noble metal catalysts under proper conditions in both cases. Fig. 1 illustrates the reaction rate r’ obtained on platinum foil at three temperatures, as a function of the partial pressure of carbon monoxide. The different (constant) values of the oxygen partial pressure are indicated in the upper part of the figure. The solid line represents the rate of impingement of the carbon
Fig. 1. Plot of T’ as a function of pco
over platinum foil.
Table 1 Data for supported catalysts Catalyst Pt-0.0 1/ZrO, Pt-l.S/Zro, Pt-lO/Zro, Pt-O.Ol/Al,O, Pt-0.2/Pyrex Pt-lS/Pyrex Pt-0.2/Quartz a Not measurable.
m4 (mg)
0.0089 1.5 10.4 1.0 0.19 1.95 0.135
Fsw (mm’)
or the
(1)
rBcoF
2. Experimental
platelet
Fmctal (cm’)
D (%)
L (mm)
210
_a
_a
392 392 330 381 457 310
30 50 50 -8 30 _a
0.72 0.17 13 _a 0.54 _a
10.0 14.0 14.0 12.5 14.4 14.4 13.0
365
S. Fuchs et al. 1 Chemical Engineering and Processing 33 (1994) 363-369
lo-'
r’ 2
mol~cm-
-se I
10-5
.
x+
. .
/
-!a?zof poz over platinum
foil.
monoxide molecules onto the surface, and quantifies the upper limit of Y’ if every impinging molecule reacted. Fig. 2 shows a similar representation of r’ as a function of the oxygen partial pressure. Due to the stoichiometry defined by Eq. (3), the rate of impingement has been multiplied by a factor of two in order to obtain the upper rate limit represented by the solid line. It is clear from these figures that the well-known formal kinetics for CO oxidation on noble metals remained valid over the whole range of pressures indicated. In oxidizing gas mixtures the rate is first order with respect to pcO and independent of pO,. The temperature dependence was very small over the range 300 < T(C) G 600. In reducing gas mixtures the order of reaction with respect to oxygen was + 1; at sufficiently high temperatures the rate was independent of pcO but was inhibited by the CO excess at ‘low’ temperatures. The reaction order changed at a well-defined value of the partial pressure ratio pco/poz which increased from ‘low’ to ‘high’ values of the total pressure in the system. However, the results indicate a major difference in catalytic activity between the ‘low’ and ‘high’ total pressure region. This is best shown by comparison of the reaction probabilities bi, i.e. ratio of the number of reacting and impinging molecules of species i. At partial pressures pco < 0.1 mbar, bco N 0.13, but over the range pcO > 0.1 mbar a much lower value of bco u 1.3 x 10d3 was obtained. The results were quite similar in the case of palladium and rhodium. This apparent ‘gap’ between ‘high’ and ‘low’ pressure results is well known. Figs. 3 and 4 show a compilation of reliable literature data from platinum and palladium catalysts together with our results at values of CO partial pressure covering a range of 10 orders of magnitude. Our own values obtained at pcO i 5 x 10m2mbar agree with the ultra-high vacuum results of White et al. [6, 151 and Tamaru et al. [ 161 (pcO < 10m5mbar). The same applies to the results reported by Coulston and Haller [7] for pcO = 5 mbar. In oxidizing gas mixtures, 13% (Pt) relative to 15% (Pd) of the impinging CO
0
,
Pm mbar
mbar
Fig. 2. Plot of r’ as a function
PO,
Fig. 3. Plot of r’ as a function ofp Co over different platinum catalysts.
10-4
rl molem-
1
*sI
:*
.
10-s
P'o,.O~'
. . ;/’
..
PC0
mbar
Fig. 4. Plot of r’ as a function catalysts.
of pco
over different
palladium
molecules react to form COZ whereas in reducing gas mixtures 10% (Pt) relative to 25O/u(Pd) of the impinging 0, molecules react to form CO,. However, over the range pcO > 5 x lo-* mbar the catalytic activities reported vary over approximately five orders of magnitude. The highest activities were obtained on foils (Pt) or single crystals (Pd), but the corresponding reaction probabilities remained approximately two orders of magnitude below those obtained at low pressures. Similar findings were obtained in the case of rhodium. What are the reasons for the dramatic differences observed between the ‘low’ and ‘high’ pressure results, a fact known equally for the HZ/O, reaction on noble metal catalysts [22-24]? Several suggestions have been made in the literature: (1) Different mechanisms for ‘low’ and ‘high’ pressures [25-321. (2) Different phase compositions (especially of oxides or surface oxides) [33-391. (3) Influence of the support [40-421. (4) Influence of surface structure (particle size) [lo, 14, 30-32,43-511. (5) Influence of contaminants [ 131. To reduce the number of influencing variables, our experiments were initially restricted to polycrystalline
S. Fuchs et al. 1 Chemical Engineering and Processing 33 (1994) 363-369
366
lO-“oW Fig. 5. Plot of r’ as a function
JQ_ mbar
100 mbor
of pco over Pt, Pd and Rh foils
foils. Contamination at higher pressures was ruled out by the use of a jet-stirred loop reactor without moving parts [5]. As a result, the observed activities remained constant for more than 1 week. Additionally, the simple geometry of the catalysts and the well-defined gas flow made it easy to account for mass-transport limitations, a fact often neglected in studies reported in the literature. Fig. 5 shows the measured reaction rates obtained over different noble metal foils as well as the calculated lines for maximum molar flow to the catalyst surface, i.e. 1 ~
F SO
(4)
4. max = e:, max = bi, bulk
of component i (CO; where ci, bulk is the concentration 0,) in the bulk of the gas phase and /I is an appropriate mass-transfer coefficient obtained via the Sherwood number [5]. Thus the horizontal line corresponds to constant maximum flow of oxygen and the inclined line corresponds to the maximum flow of carbon monoxide which is dependent upon the CO partial pressure. It is obvious that the reaction rates measured are limited by mass transfer. This assumption can be verified by varying the characteristic length L of the catalysts as shown in Fig. 6 for the different platinum catalysts. Even if the value of L is reduced from 10 mm to 0.16 mm, i.e. /I is enhanced from 29 to 205 cm s-l the measured reaction rates remain mass transfer limited.
Fig. 7. Plot of r’ as a function structure.
of pco:
”
Fig. 6. Plot of r’ as a function lengths.
'.,,.,I 1
of pco:
",,',.I PO0 mbor
variation
10
of characteristic
and
This striking but somewhat trivial result is not altered by the use of various supported catalysts (Fig. 7) even if the surface of the supported platelet is not totally covered by the noble metal (catalyst Pt-O.Ol/ZrO,). The conclusion also remains valid at lower temperatures; even if the temperature was decreased to 180 “C (at lower temperatures the conversions in the recirculation system were too low to be accurately determined), the measured rates were clearly transport limited as long as they remained first order with respect to the limiting component. The results thus indicate that the sink at the surface created by the chemical reaction is sufficiently strong to make the reaction transport limited in every case. It is interesting to note that the case of the supported catalyst which is uncompletely covered by the active component has been investigated theoretically and experimentally by Schhinder et al. [ 52, 531 for H, oxidation on platinum. It follows, therefore, that any evaluation of the results must take into account the coupling of mass transfer and chemical reaction. If both steps are first order dependent in concentration, i.e. over the range where the reaction rate depends linearly on the limiting component, the intrinsic rate constant k’ can be calculated from the experimentally determined value k:,, and the estimated transport coefficient p via x cc0 -k’ - exp cc0
lo-'! 0.1
influence of support
(5)
The calculated values of either k’ or b,, represent only lower limits but the experimental results obtained on the catalysts with the smallest characteristic length, i.e. the highest values of /?, lead to values of b,, which compare favourably to the sticking probabilities for CO absorption determined under high vacuum conditions (Pt wire, L =O.l6mm, b,= 0.37; Pd wire, L = 0.16 mm, bco = 0.24; Rh foil, L = 10 mm, bco = lo-‘). This result clearly indicates that under oxidizing conditions and at temperatures above 180 “C, at least in the case of Pt and Pd, the catalytic activity is controlled by
S. Fuchs et al. 1 Chemical Engineering and Processing 33 (1994) 363-369
the same elementary steps as in the ultra-high vacuum model experiments, viz. the rate of adsorption of the limiting compound CO. In the case of rhodium, the interpretation of the results is less clear but the rates obtained are undoubtedly higher than those reported by Goodman et al. [4,9, 54, 551 for single crystal surfaces. Under a reducing atmosphere and at low temperatures, i.e. the region of inhibition by CO, mass-transfer resistances do not interfere with the measured rates. Over that range, it is therefore possible to compare the results obtained with catalysts of various structure and support. If the rates can be related to the accessible surface of the noble metal, the activities measured on all catalysts of the same metal coincide within a factor of two, Thus no significant influence of catalyst structure and support has been detected. In this context it is interesting to look at the elementary reaction sequence of the CO oxidation on noble metals. It is generally accepted that CO oxidation proceeds via a Langmuir-Hinshelwood step, i.e. the reaction rate is proportional to the surface coverage by both carbon monoxide (O,,) and oxygen (Go), i.e. r LH
=
%etai ~ . kLH a,-, N
e.
(6)
= kzH exp g (7) ( > From a knowledge of the intrinsic rate, it is possible to specify a lower limit for the Langmuir-Hinshelwood most workers have rate coefficient kLH. Unfortunately only been interested in activation energies and the pre-exponential factor k% merely as an adjustable parameter. It is therefore relatively difficult to compare the limiting value of ktH estimated in our study to the literature values. However, it can be stated that for the usual range of activation energies reported, i.e 90 < ELH (kJ mall’) < 130, the corresponding values of the pre-exponential factor must lie in the range 4 x 10” < ktH (SC’) < 2 x 1016. k,,
4. Conclusions
Detailed measurements of CO oxidation by oxygen on various noble metal catalysts over a broad range of pressure and temperature lead to two important conclusions: (1) Over the range of gas composition and temperature where the rate of reaction is zero order with respect to the component in excess and first order with respect to the limiting reactant, the intrinsic rate coefficients determined under ultra-high vacuum conditions remain valid. (2) Over the range of CO inhibition, neither the catalyst structure nor support have any influence on the reaction, at least in the case of platinum.
367
Thus the apparent gap between the rates of CO oxidation obtained in model experiments and under ambient pressure must be attributed to the influence of mass-transfer limitations.
Nomenclature b
r
reaction probability, concentration, mol mm3 dispersion, activation energy, kJ mol-’ area, m2 rate coefficient length, m mass, kg density of surface metal atoms (1.25 x 10” m-‘) Loschmidt number (6.023 x 10z3 mol-‘) partial pressure, Pa total pressure, Pa reaction rate, mol cmm2 s-i
P 0
mass-transfer coefficient, m s-i surface coverage, -
L E F k L
m n
N P
P
Subscripts geo i
LH
geometric arbitrary species i Langmuir -Hinshelwood
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